Hydrocarbons known as crude oil are found in the world usually retained in sandstones of different porosities. The reservoirs lay from a few meters to several thousand meters below the earth surface and the seabottom, and vary largely in size and complexity, with respect to their fluid and gas contents, pressures and temperatures.
Petroleum is produced by means of wells drilled into the formations. The well itself is a complicated construction, including casings which protect the well bore against the formation itself and the pressures exerted by the reservoir fluids. Depending upon the depth, the casings are subjected to a stepwise reduction in diameter. In other words, pipe diameter decreases as depth increases. It is not unusual to have 50" (127 cm) casing in the upper regions and 7.5" (19,05 cm) casing in the lower ones.
Petroleum itself is drained from the productive formation by means of holes drilled in the casing, being, thereafter, lifted to the surface through which is referred to as production tubing. This tubing is centralized inside the casing by means of special centralizers, so that an annulus exists between the producting tubing and the casing.
Petroleum is initially produced due to the original reservoir pressure being higher than the complex forces of fluid adherence to the porous media. As pressure decreases in the course of production, a point of equilibrium is reached in which the adhesion forces are higher than the remaining pressure in place. At this point most part of the petroleum is still in the reservoir. It is estimated, in a global average, to be equal to nearly 85% of the petroleum which was there initially, but the recovery indexes vary largely from one reservoir to another. As an example we mention the Ekofish field, in the North Sea, where the primary recovery index was 17% of the original oil in place (OOIP), and the Statfjord, where said index is estimated in 45% of OOIP.
The object of all methods designed to improve petroleum recovery is, therefore, that of trying to overcome those adherences. The theoretical base to explain the cause of those adherences is as follows:
A--forces due to wettability PA0 B--forces due to permeability PA0 C--capillary forces PA0 D--adhesive and cohesive forces
It is convenient that the adherence forces dealt with in this invention be explained more in detail.